(Xinhua)

The municipal meteorological station issued the city's first orange fog warning Sunday morning due to decreased visibility caused by the heavy smog.

Heavy fog envelops Beijing on Sunday morning and the municipal meteorological station issues the city's first orange fog warning. [Photo/Xinhua]

Monitoring data released on Sunday showed that air quality indexes in most regions of Beijing had hit 500, the indexes' highest level.

The municipal environmental monitoring center said readings for PM2.5, or airborne particles measuring less than 2.5 micrometers in diameter, had reached more than 700 micrograms per square meter at several monitoring stations in Beijing, reaching as high as 993 Saturday evening.

"These figures represent extremely bad pollution. Pollutants have gradually accumulated over the course of recent windless days, making the air quality even worse," said Zhu Tong, a professor from the college of environmental sciences and engineering at Peking University.

The pollution is expected to engulf Beijing until Wednesday, when wind will arrive to blow the smog away, according to a weather report from the meteorological station.